It’s a holdover from the early days of Rainbow Six would be my guess. You had like a dozen or so operatives with mild differentiations in stats and traits. Each guy had a “service record” of sorts, which gave a little more context for what was, in essence, the games’ lives system. If I remember right, some of these names were either pulled from or incorporated into the Tom Clancy Universe of novels and adaptations. It was a practically free way to inject some story and character into games that were pretty light on those details otherwise.

Of course, modern Rainbow Six has no need for these things, but inertia is a bitch, and you can be sure some grognards would piss and moan about a “feature” being removed if they stopped including these details.